The present invention relates to an air lock for introducing and continuously passing a strip of substrate, such as a metal sheet, into and through a vacuum chamber, comprising an enclosure in which there are mounted rollers which can rotate about parallel axes and between which the said strip can travel.
U.S. Pat. No. 3,868,106 discloses an air lock wherein all the rollers are aligned with respect to each other. The sealing is achieved by placing shoes with a slight clearance almost in contact with the strip. The strip is placed in contact with each of the rollers over an angle of 180 degrees. This device suffers from the drawback that it takes a lot of space. In addition, the presence of the shoes at opposite sides of each of the rollers in order to guarantee air-tightness between the chambers renders the device relatively complex. Due to the large angle of contact, it is difficult for applying a new strip in the passage between the rollers.
U.S. Pat. No. 2,972,330 discloses an air lock wherein the strip is in contact with each of the rollers over an angle of 180 degrees. The air-tightness between the regions is again guaranteed due to the presence of shoes. This device suffers therefore also from the drawbacks of U.S. Pat. No. 3,868,106.
One of the objects of the invention is to present an air lock which makes it possible, simultaneously, to achieve a minimum size and maximum air-tightness for the continuous passage of a strip of substrate.
To this end, according to the invention, the aforementioned enclosure contains at least three successive rollers extending in the direction of travel of the strip and between which there is a passage for the latter, the first and third rollers being located on one side of this passage whilst the second roller is arranged on the opposite side of this passage with respect to the two aforementioned rollers, and means allowing an essentially airtight region to be formed between a strip travelling through the said passage and the first and third rollers, this region being connected to a vacuum pump, other means being provided for adjusting the pressure with which the strip rests on these rollers.